Paper Product with Increased Relative Wet Tensile Strength and Softness, Method for Production and Use Thereof

ABSTRACT

The present invention relates to flat paper products having an increased relative wet strength and softness, a method for the production of the same and also the use of paper products of this type in the form of tissue products. This is achieved by a cross-linkage of the cellulose fibres contained in the paper product with a cationic graft copolymer which is constructed on the basis of polyethyleneoxide- or polyethyleneglycol segments and polyethyleneimine segments.

The present invention relates to flat paper products having increased relative wet strength and softness, a method for the production of the same and the use of paper products of this type in the form of tissue products. This is achieved by cross-linkage of the cellulose fibres contained in the paper product with a cationic graft copolymer which is constructed on the basis of polyethyleneoxide- or polyethyleneglycol segments and polyethyleneimine segments.

The mutual cross-linkage of cellulose fibres in the paper production process is crucial for the quality and the properties of the produced paper. Without adhesives, cellulose fibres mainly form only hydrogen bonds at the intersection points. Such paper has no wet strength and has only a restricted elasticity or softness. There is understood by wet strength the strength of papers in the completely saturated state. In order to improve the adhesion of the cellulose fibres to each other and hence the usage properties of the paper, polymer adhesives inter alia on an epichlorohydrin base or on a polyacrylamide base are used worldwide as so-called wet strength agents.

The cellulose fibres used during paper production are negatively charged. For a simple and economical paper production method which is known to the person skilled in the art, it is most favourable if treatment chemicals, e.g. wet strength agents, are metered directly into the aqueous pulp stock with which the paper is then produced. The industrial tissue production process may be explained subsequently in brief.

The aqueous pulp stock is then passed during the industrial process to the machine wire(s), is formed there and partly drained and passes subsequently into the dry part of the tissue machine. In the dry part there is the so-called steam-heated yankee cylinder which has a surface temperature of 80 to 140° C. In addition, one or more gas-heated hoods can be situated thereabove through which hoods hot air is blown onto the tissue web. The air is heated for this purpose to temperatures of 200 to 750° C. During an extremely short contact time of a few ms on the cylinder, the hardening process of the wet strength agents begins which is concluded during the subsequent storage of the finished tissue web, the so-called subsequent ripening.

In order that a potential wet strength agent can be absorbed onto the cellulose fibre during application on the pulp stock, said agent is advantageously water-soluble or water-dispersible and cationic since the cellulose fibres used during paper production are negatively charged. Therefore the use of cationic polymers as wet strength agents in the paper or tissue industry, which are based in part on very different chemical structurings such as polyamides or polyacrylamides, is regarded as current state of the art. These wet strength agents are normally added in quantities of 8 to 10 kg/tonne for household tissues. The cellulose fibres of the paper or tissues hold together in the dry state in the network of a sheet by means of fibre-fibre contact points which are based on van der Waals or hydrogen bonds. These bonds are very sensitive to water, i.e. the more moist the tissue becomes, the looser these bonds become. In order to be able to produce so-called wet strength papers, such as e.g. kitchen or household towel or toilet tissue, wet strength agents are added which have the task of forming bonds which are at least temporarily resistant to water. According to the current state of the art, various chemically based polymer wet strength agents are available in paper or tissue production which are described in the relevant literature, e.g. in “Papermaking Chemistry”, Book 4, Ed. Leo Neimo, pp. 288-301. There are used predominantly, melamine-formaldehyde resins (MF) (U.S. Pat. No. 4,461,858) and cationic polymers based on polyamide-epichlorohydrin (PAE) and polyamidoamine-epichlorohydrin (PAAE) (U.S. Pat. No. 2,926,116, U.S. Pat. No. 2,926,154, U.S. Pat. No. 3,733,290, U.S. Pat. No. 4,566,943, U.S. Pat. No. 4,605,702).

A disadvantage with the above-mentioned wet strength agents is, on the one hand, that the treated tissue does in fact have increased wet strength but has reduced softness. The sought softness must then be achieved by an additional mechanical treatment of the tissue. A further disadvantage of the PAEs is the production-caused content of organic halogen compounds. WO 00/40639 describes a PAE-based wet strength agent with a low content of organically bonded chlorine. Furthermore, water-dispersible wet strength agents based on polyisocyanate are described in DE 196 40 205 A1, which are obtained by conversion of the initial components polyisocyanate, polyalkyleneoxide polyether alcohol, of a quaternised aminopolyalkyleneoxide polyether alcohol and also possibly further auxiliary materials and additives. Furthermore, DE 698 14 359 T2 teaches that e.g. polyethyleneimine belongs to the temporary wet strength agents.

Furthermore, it is known to the person skilled in the art that polymers which have a low glass transition point, i.e. ≦RT, have soft properties. DE 689 16 860 describes in detail a method for the production of absorbent structures in which the absorbing structures are produced from mixed paper raw materials, of which one is treated with a latex with an elastomer core. The softness is hereby achieved by the latex with the low glass transition point. In order that the latices are absorbed onto the cellulose fibre in the wet method, the latter have a polymer shell based on oleyl polyethoxylate which carries a quaternary functional (trimethyl) ammonium group at the end of the ethoxylate chain. It is however disadvantageous with this method that the soft latices are of a hydrophobic nature and hence have a negative influence on the absorption capacity of the paper produced therefrom in a single “one batch” application. Hence with this method various pulp stocks must be worked with, only one of which is treated with the latex. In a further step, the pulp stock treated with latex is mixed with an untreated fibre suspension in order thus to achieve the water-absorbing effect of the paper. WO 96/33310 describes a method for the production of soft-creped tissue which is obtained by a specially controlled production process, mainly by mechanical treatment.

It is therefore the object of the present invention to develop a wet strength agent which overcomes the above-mentioned problems. The wet strength should thereby be increased without having negative effects on the softness. At the same time, the absorption of the paper should also not be negatively affected. Furthermore, the application of chemicals should be effected in as simple a manner as possible directly in the pulp or during production of the paper product. Multistage methods, such as e.g. mixing of treated pulp with untreated pulp as described for example in DE 689 16 860 T2, should however be avoided. Furthermore, the use of organically bonded halogens in the treatment chemicals should be avoided.

This object is achieved by the paper product having the features of claim 1 and the method for the production of the paper product having the features of claim 12. The further dependent claims reveal advantageous developments. In claim 23, the use of the paper product according to the invention is described.

According to the invention, a paper product with increased relative wet strength and softness is provided, which contains cellulose fibres which are cross-linked with a graft copolymer comprising a polyethyleneimine segment of the general formula I,

x, y and z being chosen such that the molar mass of the polyethyleneimine segment is in the range of 1000 to 2000000 dalton, and also at least one polyalkyleneoxide- and/or at least one polyalkyleneglycol segment of the general formula II,

with R₁ and R₂ independently of each other C₁ to C₁₂-alkyl, n being chosen such that the molar mass of the polyalkyleneoxide- or polyalkyleneglycol segment is in the range of 350 to 2000000 dalton.

It was found surprisingly that by applying graft copolymerisation in the aqueous pulp stock which comprises branched or linear polyethyleneimine (PEI) on which polyalkyleneglycol- or polyalkyleneoxide segments were grafted, i.e. were cross-linked chemically with the N-atoms of the PEI by covalent bonding, the above-mentioned problems can be overcome. Alternatively, the above-mentioned problems can be overcome by the application of graft copolymerisation in the aqueous pulp stock which comprises polyalkyleneglycol- or polyalkyleneoxide segments onto which polyethyleneimine was grafted. The above-mentioned problems are however not overcome if mixtures of polyalkyleneglycol homopolymer and PEI homopolymer or only polyalkyleneglycol homopolymer or only PEI homopolymer are used. The effect according to the invention is achieved only by the grafted polymer structure.

Preferably the backbone of the graft copolymer comprises polyethyleneimine, there being bonded to the latter grafts comprising at least one polyalkyleneoxide and/or at least one polyalkyleneglycol. Another variant provides that the graft copolymer has a backbone comprising at least one polyalkyleneoxide and/or at least one polyalkyleneglycol, grafts comprising polyethyleneimine then being present.

Preferably the graft copolymer comprises polyethyleneimine with a molar mass in the range of 20000 to 1000000 dalton.

Preferably polyethyleneglycol is used as polyalkyleneglycol or preferably polyethyleneoxide as polyalkyleneoxide. These thereby have preferably a molar mass in the range of 350 to 1000000 dalton, particularly preferred of 350 to 80000 dalton.

A terminal hydroxyl group of the polyalkyleneoxide and/or polyalkyleneglycol is preferably blocked. Preferably the terminal hydroxy group is thereby blocked with a methoxy, ethoxy, propyloxy, butyloxy and/or benzyloxy group. The free terminal hydroxyl group of the polyalkyleneglycol or polyalkyleneoxide is converted into a chemical-reactive group, i.e. by reaction with epihalogenhydrin by forming a polyethyleneglycolglycidyl ether. Polyalkyleneglycols modified in this manner then form, by chemical reaction with the primary and secondary amine groups of the polyethyleneimine, the graft copolymers which are desired for the paper products according to the invention.

According to the invention, likewise a method for the production of flat paper products with increased relative wet strength and softness is provided from a pulp stock, in which there is added to the pulp stock a graft copolymer comprising a polyethyleneimine segment of the general formula I

x, y, z being chosen such that the molar mass is in the range of 1000 to 2000000 dalton, and also at least one polyalkyleneoxide- and/or at least one polyalkyleneglycol segment of the general formula II

with R₁ and R₂ independently of each other C₁ to C₁₂-alkyl and n being chosen such that the molar mass thereof is in the range of 350 to 2000000 dalton, and/or the paper product is treated during production thereof or subsequently with the graft copolymer.

Polyalkyleneglycols can be coupled with polyethyleneimine by various methods in order thus to obtain the desired graft copolymers. Sung et al. (Biol. Pharm, Bull. 26(4), 492-500 (2003) describe the synthesis of polyalkyleneglycols grafted on PEI. They begin thereby in the synthesis of a polyethyleneglycol (MPEG) which is terminated on one side with a methyl group and is converted with epichlorohydrin with base effect into an epoxy-terminated MPEG. This epoxy-terminated MPEG is grafted onto the PEI in a further synthesis step so that a graft copolymer is obtained. By varying the MPEG molecular weights which are used and also from the ratio of epoxy-terminated MPEG to PEI, different structures with different graft degrees and lengths of the graft branches can be synthesised.

A further method of synthesising graft copolymers based on polyalkyleneglycol-polyethyleneimine resides in bonding a polyalkyleneglycol carboxylic acid, by the coupling methods currently known from peptide chemistry, to the primary or secondary amine groups of the PEI by linkage of peptide bonds. The current coupling methods, e.g. with the help of carbodiimides are described for example in “Amino acids, Peptides, Proteins”, H. Jakubke, H. Jeschkeit, Licensed Edition for the Chemistry Press, Weinheim, 1982, ISBN 3-527-25892-2. More recent coupling methods with water-soluble coupling reagents, such as e.g. N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride are described for example in Sheehan et al. (J. Am. Chem. Soc. 95, 875 (1973)), Nozaki et al. (Bull. Chem. Soc. Jpn. 55, 2165 (1982)) and Schmidt et al. (J. Chem. Soc. Chem. Commun. 1687 (1992)).

It is also possible with the above-mentioned methods to graft polyethyleneimines onto polyethyleneglycols. This is possible for example by using bifunctional polyethyleneglycols, such as e.g. polyethyleneglycol diacids or polyethyleneglycolbisglycidyl ether.

Particularly preferred are coupling methods with activators which permit coupling under aqueous conditions, such as e.g. N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride. The coupling can be implemented in principle also with organic water-free solvents by means of carbodiimides and carbonyl diimidazoles.

The above-mentioned graft copolymer is added as an additive to a paper product, in particular to a so-called tissue product. Further paper products, to which the described graft copolymer can be added as additive are inter alia graph paper, newspaper, cardboard, copy paper and special papers, such as for example banknotes or even filter papers. A so-called tissue paper, which tissue products comprise, differs from normal papers in particular by its very low basis weight of normally less than 40 g/m².

In general there are described as “tissue papers” or better as raw tissue papers the single-layer intermediate products, which come from the paper machine, comprising light papers, i.e. produced with a low basis weight, which were dry-creped as a rule on a so-called yankee cylinder with the help of a crepe scraper. The single-layer raw tissue comprising respectively one or more layers can thereby be constructed.

There are termed as “tissue products” all the single or multilayer end products which are produced from raw tissue and orientated to the requirements of the end user, i.e. are manufactured with the most varied of requirement profiles.

Typical properties of tissue papers are the good capacity to absorb strength energy, their drapability, good textile-like flexibility, properties which are often termed as crumple-softness, high surface softness, a high specific volume with a tactile thickness, as high a liquid absorption capacity as possible and, according to the application, a suitable wet and dry strength and also an interesting optical appearance of the outer product surface. Because of these properties, tissue papers are processed into tissue products (tissue paper products) and are then available to the end user in the most varied of forms and assemblies, for example as wipes, handkerchiefs, household towels, in particular as kitchen towels, as sanitary products, (e.g. toilet papers), as paper tissues, cosmetic tissues or serviettes.

For successful use of tissue products in the most varied of application areas, dependent upon the purpose of use, frequently different and partly contradictory properties are required.

Tissue papers are produced nowadays as a rule by three different methods. These methods are:

-   -   the wet crepe process     -   the dry crepe process and     -   the through air drying process

The methods differ as a result of the construction of the tissue machine. In the case of the through air drying process, the press for mechanical draining of the tissue web is therefore replaced by throughflow cylinders through which hot air is blown (energetic drying). Also the construction of the so-called wire part, in which the web is formed and partially drained, can be different. The sheet formation can be formed on only one wire (breast roller former) between two wires (C-wrap or S-wrap former) or between one wire and a felt (crescent former).

Despite these differences, all the plants have a similarly constructed so-called constant part. This differs mainly in whether the material inlet is operated with only one or several layers. If multilayer tissue papers are produced according to the so-called multilayer method, some units, such as e.g. the material inlet pump, must be present whenever the finished tissue paper is intended to have layers.

The constant part of a tissue machine begins with the pulper in which dried pulp is dissolved or with a stacked tower in which up to several hundred cubic meters of pulp stock can be stored and ends at the material inlet. Since in a plurality of circulations material-water suspensions are returned from the machine to different points in the constant part, these circulations also fall in the constant part region (e.g. wire water or machine rejects). Pulpers are used if dried pulp is to be broken up. This pulp is delivered as a rule from an external pulp factory. Stacked towers are used if pulp is produced in the same works (integrated factory) or waste paper is processed. These two raw materials are then not dried before use on the tissue but only drained up to a material density of maximum 25% (250 g fibres in 1 l water) in order to separate the circulations of the tissue machine and the pulp factory or waste paper processing.

In tissue production, the most varied of pulps or types of waste paper can be used as fibre material. Both sulphate and sulphite pulps are used. The bleaching can be implemented without chlorine or with chlorine-containing chemicals, such as e.g. hypochlorite. The pulp is generally produced from different woods (deciduous and coniferous woods). Other fibrous materials, such as e.g. CTMP (Chemical Thermo Mechanical Pulp—chemical-mechanical wood material), can likewise be used. The fibrous materials can be used individually or mixed. If a multilayer material inlet is used, generally different fibrous material is used in all the different layers. Hence in the different legs of the constant part, different units can be used.

The fibrous material has both an influence on the units used in the constant part of the tissue machine and on the metering point for the chemicals which are used. Dependent upon the type of fibre e.g. more or fewer cleaning units must be used (thick material cleaner, cleaner, pressure sorter etc.). Also refiners are used differently according to the type of fibre.

The tissue production is assisted, improved or controlled by the use of chemicals. Normally a difference is thereby made between process chemicals, functional chemicals, coating chemicals for the yankee cylinder and chemicals for cleaning.

Process chemicals are inter alia pH regulators, defoamers, retention and flocculation aids for improving the fibre retention during the sheet formation or for coagulating fibres in a disc filter or a microflotation, disruptive material fixers for binding undesired particles in the system and also biocides for attacking bacteria and for avoiding the formation of slime.

Functional chemicals serve inter alia as wet strength agents for increasing the strength of the wet tissue paper, as dry strength agents for increasing the strength of the dry tissue paper, as so-called softener/debonders for improving the surface softness and for reducing the stiffness of the tissue paper and also as colourants and optical brighteners for increasing the degree of whiteness.

Coating chemicals for the yankee cylinder are used inter alia for controlling the adhesion of the tissue paper on the yankee cylinder, the number of chemicals used being able to vary normally between one and for instance five.

Suitable chemicals for cleaning are inter alia organic and inorganic acids and caustic sodas. These are normally used for cleaning the wires and felts in the tissue machine.

The location of use of chemicals of this type and hence also the graft copolymers to be used according to the invention as wet strength agents within the tissue machine can be very different. Normally however the functional chemicals and hence also the described graft copolymers are intended to be metered at a point at which, directly after their metering, good mixing with the fibre suspension is effected. Suitable points of the tissue machine are therefore inter alia the rapid mixer, the material level box or connection pipes directly in front of pumps. Further possible addition locations are situated both in the wet part of the tissue machine, at the end of the wire part, in front of or within the press part and also in the dry part disposed after the press part. Furthermore the possibility also exists of applying treatment chemicals, such as e.g. wet strength agents, by spray application onto the yankee cylinder. The addition of such treatment chemicals can be effected also on the Pope roller with production of a treatment agent film which is subsequently transferred to the tissue web during the rolling process. Treatment chemicals can be added, also within doubling machines or within processing machines, onto the outer layers of the tissue paper or of the tissue product.

The metering point is not only dependent upon the units contained in the constant part but also upon the fibrous material which is used, the process water quality (pH value, water hardness, conductivity etc.) and also upon the use of different chemicals on the same tissue machine. If for example wet strength agents are used together with dry strength agents and/or softeners, it must be ensured that both or all three chemicals can be absorbed onto the fibre and the metering points must be chosen such that the chemicals cannot react with each other before they are fixed on the fibres.

Also the reaction time with the fibrous material can be of different lengths. Hence the metering point can be nearer and further removed from the material inlet according to the fibrous material which is used.

If the chemicals which are used are sensitive to shear stress, it must be ensured that no units are situated between the metering point and the material inlet in which the chemical can again be sheared off from the fibres.

The subject according to the application is intended to be explained in more detail with reference to the subsequent examples without restricting said subject to the examples shown here.

FIG. 1 shows the basis weight of the graft copolymers produced according to the examples, in comparison with a wet strength agent of the of the art.

FIG. 2 shows the breaking length in the dry state of the graft copolymers according to examples 2 to 9 in comparison with a wet strength agent of the state of the art.

FIG. 3 shows the breaking length in the wet state of the graft copolymers according to examples 2 to 9 in comparison with a wet strength agent of the state of the art.

FIG. 4 shows the relative wet moisture of the graft copolymers according to examples 2 to 9 in comparison with a wet strength agent of the state of the art.

EXAMPLE 1 Synthesis of Epoxy-Terminated methoxy-poly(ethyleneglycol)-glycidyl ether (MPEG)

Polyethyleneglycol monomethyl ether (Fluka, 35 g; M=350 g/mol; 0.1 mol) was added to a mixture comprising epichlorohydrin (Aldrich, 27.76 g; 0.3 mol; M=92.53 g/mol, b.p.: 115-117°), sodium hydroxide (3.2 g; 0.08 mol; M=40 g/mol) and water (0.5 ml) and heated for 16 h at 60° C. under an argon atmosphere. Heating took place thereafter for 1.5 h at 90° C.

The reaction mixture was dissolved in 150 ml chloroform and mixed with 100 ml water. By adding sodium dihydrogen phosphate (NaH₂PO₄), the aqueous phase was adjusted to a pH value of 7. The precipitated solid material was filtered off and discarded. After separating the aqueous phase, the organic phase was washed twice with 50 ml water and subsequently dried with sodium sulphate. Subsequently, the chloroform was distilled off in the vacuum. Epichlorohydrin residue remaining in the product was removed in the high vacuum at 10⁻²-10⁻³ mbar. In order to check the complete removal of epichlorohydrin, a sample was heated in the aqueous, basic medium then acidified with nitric acid and mixed with silver nitrate. The test was negative (no precipitation of silver chloride), which means the absence of halogen in the product.

EXAMPLE 2 Synthesis of PEI-Graft-PEG

There was added to a solution of methoxy-poly(ethyleneglycol)-glycidyl ether (from example 1) (12.0 g; 0.03 mol; Mw=400 g/mol) in 10 ml methanol a solution of polyethyleneimine (Aldrich, Mw=40000 g/mol, 14.9 g, 0.0006 mol) in 30 ml methanol. The homogeneous mixture was agitated for 12 h under reflux. Subsequently the reaction mixture was purified by means of dialysis against water (dialysis hose ZelluTrans by Roth, MWCO 4000-6000 g/mol). The resulting graft copolymer was isolated by freeze-drying.

EXAMPLE 3 Synthesis of PEI-Graft-PEG

There was added to a solution of methoxy-poly(ethyleneglycol)-glycidyl ether (from example 1) (10.0 g; 0.025 mol; Mw=400 g/mol) in 10 ml methanol a solution of polyethyleneimine (Aldrich, Mw=40000 g/mol, 20.0 g, 0.0005 mol) in 30 ml methanol. The homogeneous mixture was agitated for 12 h under reflux. Subsequently the reaction mixture was purified by means of dialysis against water (dialysis hose ZelluTrans by Roth, MWCO 4000-6000 g/mol). The resulting graft copolymer was isolated by freeze-drying.

EXAMPLE 4 Synthesis of PEI-Graft-PEG (PEI-PEG 3)

There was added to a solution of methoxy-poly(ethyleneglycol)-glycidyl ether (from example 1) (0.4 g; 0.001 mol; Mw=400 g/mol) in 10 ml methanol a solution of polyethyleneimine (Aldrich, Mw=40000 g/mol, 8.0 g, 0.0002 mol) in 30 ml methanol. The homogeneous mixture was agitated for 12 h under reflux. Subsequently the reaction mixture was purified by means of dialysis against water (dialysis hose ZelluTrans by Roth, MWCO 4000-6000 g/mol). The resulting graft copolymer was isolated by freeze-drying.

Yield: 74% of the theoretical

EXAMPLE 5 PEI-PEG 7

There was added to a solution of methoxy-poly(ethyleneglycol)-glycidyl ether (from example 1) (0.88 g; 0.0022 mol; Mw=400 g/mol) in 10 ml methanol a solution of polyethyleneimine (Aldrich, Mw=40000 g/mol, 8.0 g, 0.0002 mol) in 30 ml methanol. The homogeneous mixture was agitated for 12 h under reflux. Subsequently the reaction mixture was purified by means of dialysis against water (dialysis hose ZelluTrans by Roth, MWCO 4000-6000 g/mol). The resulting graft copolymer was isolated by freeze-drying.

Yield: 90% of the theoretical

EXAMPLE 6 PEI-PEG 14

There was added to a solution of methoxy-poly(ethyleneglycol)-glycidyl ether (from example 1) (1.76 g; 0.0044 mol; Mw=400 g/mol) in 10 ml methanol a solution of polyethyleneimine (Aldrich, Mw=40000 g/mol, 8.0 g, 0.0002 mol) in 30 ml methanol. The homogeneous mixture was agitated for 12 h under reflux. Subsequently the reaction mixture was purified by means of dialysis against water (dialysis hose ZelluTrans by Roth, MWCO 4000-6000 g/mol). The resulting graft copolymer was isolated by freeze-drying.

Yield: 82% of the theoretical

EXAMPLE 7 PEI-PEG 20

There was added to a solution of methoxy-poly(ethyleneglycol)-glycidyl ether (from example 1) (6.00 g; 0.015 mol; Mw=400 g/mol) in 10 ml methanol a solution of polyethyleneimine (Aldrich, Mw=40000 g/mol, 20.0 g, 0.0005 mol) in 30 ml methanol. The homogeneous mixture was agitated for 12 h under reflux. Subsequently the reaction mixture was purified by means of dialysis against water (dialysis hose ZelluTrans by Roth, MWCO 4000-6000 g/mol). The resulting graft copolymer was isolated by freeze-drying.

Yield: 75% of the theoretical

EXAMPLE 8 PEI-PEG High Molecular

There was added to a solution of methoxy-poly(ethyleneglycol)-glycidyl ether (from example 1) (4.8 g; 0.012 mol; Mw=400 g/mol) in 10 ml of an aqueous 0.1 M NaCl solution, a solution of high-molecular polyethyleneimine (Fluka, Mw=800000 g/mol, 16.0 g of a 50% aqueous solution, 1×10⁻⁵ mol) in 30 ml of a 0.1 M aqueous NaCl solution. The homogeneous mixture was agitated for 12 h under reflux. Subsequently the reaction mixture was purified by dialysis against water (dialysis hose ZelluTrans by Roth, MWCO 4000-6000 g/mol). The resulting graft copolymer was isolated by freeze-drying.

Yield: 59% of the theoretical

EXAMPLE 9 Production of a Graft Copolymer Via Peptide Coupling of a PEG Carboxylic Acid with PEI (PEG-COOH-g-PEI)

In a 100 ml three-neck flask, 0.72 g PEG600-di-acid (Fluka, M=600 g/mol; 1.210-3 mol) and 575.1 mg N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (Merck, 310-3 mol) were dissolved in 10 ml distilled water. Subsequently the solution was mixed with 345.3 mg N-hydroxysuccinimide (Fluka, 310-3 mol). After 10 minutes, 4.0 g polyethyleneimine (Aldrich, MW=40000 g/mol; 110-4 mol) in 20 ml H₂O were added and agitated for 12 h at 20° C. Subsequently the reaction mixture was purified by dialysis against water (dialysis hose ZelluTrans by Roth, MWCO 4000-6000 g/mol). The resulting graft copolymer was isolated by freeze-drying

Yield: 55% of the theoretical

EXAMPLE 10 Sheet Formation Test with the Graft Copolymers Produced in A

The laboratory sheet formation was effected according to the rapid Köthen method. The production of the laboratory sheets was effected according to DIN EN ISO 5269-2: 1998. The rapid Köthen laboratory sheet former comprises a sheet forming device, the transition elements and a plurality of vacuum dryers. In principle a circular laboratory sheet is formed from a fibre suspension on a wire cloth by suction effect. The sheet is subsequently dried under defined conditions.

The basis weight of the formed sheet is 75±2 g/m² otro (oven dried), i.e. one sheet has a mass of 2.36 g±0.06 g.

The pulp used for the tests was non-predried spruce sulphite pulp (Pulp Factory Mannheim) with a degree of beating of 15 SR.

A pulp suspension with a material density of 0.236% was used to produce the laboratory sheets. The chemicals to be tested were added in a 2.5% solution to a litre of the pulp stock until concentrations of 3.6 and 9 kg/t were set. The reaction time with constant agitation was always 2 min, thereafter the laboratory sheet formation was effected according to the mentioned method. For the test, the laboratory sheets were conditioned to normal conditions (23° C., 50% relative humidity) (DIN EN 20187 1993).

The strength (breaking force) of paper is the force measured during the test at the moment of breakage of the sample. In order to minimise the variations in the basis weight, the breaking length is determined from the breaking force according to the following formula: breaking length=10⁶×breaking force/(basis weight×strip width×acceleration due to gravity) [m].

The samples used had a width of 15 mm, the free gripped length was 100 mm. The testing was effected following DIN EN ISO 1924-2 1994-04. Wet breaking loads were tested on papers which were immersed in advance in distilled water for 30 s (DIN ISO 3781 1994-10).

In order to determine the relative wet strength, the ratio of the breaking length in the moist state to the dry breaking length was formed:

Relative wet strength=breaking force (wet)/breaking force (dry)×100(%)

The results of the chemicals produced according to examples 2 to 9 were tested in comparison with a standard wet strength agent (PAAE) on the laboratory sheet and are presented in FIGS. 1 to 4 and in Table 1.

TABLE 1 Breaking Breaking Relative Metering Basis length length wet quantity weight dry wet strength (kg/t) (g/m²) (m) (m) (%) PAAE 3 78.0 5012 635 12.7 6 79.0 5304 955 18.0 9 78.0 5456 1088 19.9 Example 3 77.1 4832 167 3.4 2 6 77.1 5194 508 9.8 9 76.8 5130 626 12.2 Example 3 75.5 5082 213 4.2 3 6 77.1 5137 509 9.9 9 77.1 5075 661 13.0 Example 3 76.1 5272 270 5.1 4 6 75.8 5122 627 12.2 9 75.2 5274 789 15.0 Example 3 75.5 5040 339 6.7 5 6 72.9 5142 686 13.3 9 74.8 5366 731 13.6 Example 3 74.8 5274 327 6.2 6 6 74.5 5164 631 12.2 9 77.4 5409 753 13.9 Example 3 77.1 5024 281 5.6 7 6 75.5 5283 599 11.3 9 76.4 4996 643 12.9 Example 3 75.2 4990 347 7.0 8 6 75.8 5332 655 12.3 9 75.8 5463 805 14.7 Example 3 75.2 5444 484 8.9 9 6 74.5 5762 800 13.9 9 75.2 5588 1027 18.4 

1. A paper product having increased relative wet strength and softness containing cellulose fibers which are cross-linked with a graft copolymer comprising a polyethyleneimine segment of the general formula I

x, y and z being chosen such that the molar mass is in the range of 1000 to 2000000 D, and at least one of a polyalkyleneoxide segment and a polyalkyleneglycol segment of the general formula II,

with R₁ and R₂ independently of each other C₁-C₁₂-alkyl, and n being chosen such that the molar mass thereof is in the range of 350 to 2000000 D.
 2. The paper product according to claim 1 the graft copolymer has a backbone comprising polyethyleneimine and grafts comprising at least one polyalkyleneoxide and/or at least one polyalkyleneglycol.
 3. The paper product according to claim 1 wherein the graft copolymer has a backbone comprising at least one polyalkyleneoxide and/or at least one polyalkyleneglycol and grafts comprising polyethyleneimine.
 4. The paper product according to claim 1 wherein the polyethyleneimine is linear.
 5. The paper product according to claim 1 wherein the polyethyleneimine is branched.
 6. The paper product according to claim 1 wherein the polyethyleneimine has a molar mass in the range of 20000 to 1000000 D.
 7. The paper product according to claim 1 wherein the polyalkyleneglycol is polyethyleneglycol.
 8. The paper product according to claim 1 wherein the polyalkyleneoxide is polyethyleneoxide.
 9. The paper product according to claim 1 wherein the polyalkyleneglycol and/or the polyalkyleneoxide has a molar mass in the range of 350 to 1000000 D, in particular 350 to 80000 D.
 10. The paper product according to claim 1 wherein a terminal hydroxyl group of the polyalkyleneoxide and/or of the polyalkyleneglycol is blocked.
 11. The paper product according to claim 10 wherein the terminal hydroxy group is blocked with a methoxy, ethoxy, propyloxy, butyloxy and/or benzyloxy group.
 12. A method for the production of flat paper products having an increased relative wet strength and softness from a pulp stock, the method comprising at least one of adding to the pulp stock, adding to the paper product during production, and adding to the paper product after production a graft copolymer comprising a polyethyleneimine segment of the general formula I

x, y and z being chosen such that the molar mass is in the range of 1000 to 2000000 D, and at least one of a polyalkyleneoxide segment and a polyalkyleneglycol segment of the general formula II,

with R1 and R2 independently of each other C1-C12-alkyl, and n being chosen such that the molar mass thereof is in the range of 350 to 2000000 D.
 13. The method according to claim 12 wherein said at least one of adding to the pulp stock, adding to the paper product during production, and adding to the paper product after production a graft copolymer comprising a polyethyleneimine segment and at least one of a polyalkyleneoxide segment and a polyalkyleneglycol segment comprises at least one of adding to the pulp stock, adding to the paper product during production, and adding to the paper product after production a graft copolymer having a backbone comprising polyethyleneimine and grafts comprising at least one of a polyalkyleneoxide and a polyalkyleneglycol.
 14. The method according to claim 12 wherein said at least one of adding to the pulp stock, adding to the paper product during production, and adding to the paper product after production a graft copolymer comprising a polyethyleneimine segment and at least one of a polyalkyleneoxide segment and a polyalkyleneglycol segment comprises at least one of adding to the pulp stock, adding to the paper product during production, and adding to the paper product after production a graft copolymer having a backbone comprising at least one of a polyalkyleneoxide and a polyalkyleneglycol and grafts comprising polyethyleneimine.
 15. The method according to claim 12 wherein said at least one of adding to the pulp stock, adding to the paper product during production, and adding to the paper product after production a graft copolymer comprising a polyethyleneimine segment and at least one of a polyalkyleneoxide segment and a polyalkyleneglycol segment comprises adding a polyethyleneglycol segment.
 16. The method according to claim 12 wherein said at least one of adding to the pulp stock, adding to the paper product during production, and adding to the paper product after production a graft copolymer comprising a polyethyleneimine segment and at least one of a polyalkyleneoxide segment and a polyalkyleneglycol segment comprises adding a polyethyleneoxide segment.
 17. The method according to claim 12 wherein adding a graft copolymer comprising a polyethyleneimine segment comprises adding a polyethyleneimine segment having a weight, relative to the initial compounds, greater than 50% by weight.
 18. The method according to claim 17 wherein adding a graft copolymer comprising a polyethyleneimine segment comprises adding a polyethyleneimine segment having a weight, relative to the initial compounds, greater than 80% by weight.
 19. The method according to claim 17 wherein adding a graft copolymer comprising a polyethyleneimine segment comprises adding a polyethyleneimine segment having a weight, relative to the initial compounds, greater than 90% by weight.
 20. The method according to claim 12 wherein said at least one of adding to the pulp stock, adding to the paper product during production, and adding to the paper product after production a graft copolymer comprising a polyethyleneimine segment and at least one of a polyalkyleneoxide segment and a polyalkyleneglycol segment comprises at least one of adding a polyalkyleneoxide segment having a blocked terminal hydroxyl group and adding a polyalkyleneglycol segment having a blocked terminal hydroxyl group.
 21. The method according to claim 20 wherein said at least one of adding a polyalkyleneoxide segment having a blocked terminal hydroxyl group and adding a polyalkyleneglycol having a blocked terminal hydroxyl group comprises at least one of adding a polyalkyleneoxide segment having a terminal hydroxyl group blocked by one of a methoxy group, an ethoxy group, a propyloxy group, a butyloxy group and a benzyloxy group, and adding a polyalkyleneglycol having a terminal hydroxyl group blocked by one of a methoxy group, an ethoxy group, a propyloxy group, a butyloxy group and a benzyloxy group.
 22. The method according to claim 12 further comprising converting a free terminal hydroxy group of said at least one of the polyalkyleneoxide and the polyalkyleneglycol with an epihalogenhydrin into a polyethyleneglycol glycidyl ether and grafting said polyethyleneglycol glycidyl ether onto the polyethyleneimine via the amine groups.
 23. Use of the paper products according to claim 1 as tissue product. 